In typical applications for hydrocarbon production from oxygenated hydrocarbons, also known as oxygenates, the raw oxygenate feedstock is subjected to hydrodeoxygenation (HDO). Chemically, hydrodeoxygenation removes oxygen from the feedstock molecules in the presence of gaseous hydrogen and a HDO catalyst, and rejects it in the form of water. Oxygenates can have different relativities during HDO. Certain oxygenates, such as those with either low molecular weights or phenolic compounds, have a lower HDO reactivity than other oxygenates. Increasing the severity of the HDO process (e.g., increasing reaction temperature, increasing residence time, increasing hydrogen partial pressure) can be employed to insure these refractory oxygenates are converted. However, undesired side reactions such as cracking to lights and coking of the catalyst are also promoted at higher HDO severities. The final oxygenate conversion during HDO is typically a compromise between the product value lost in the form of unreacted oxygenates, and the cost savings in the form of higher overall HDO product yield and longer catalyst run times.
Following HDO, unreacted oxygenates in the initial HDO product will typically partition between the hydrocarbon and the aqueous portions of the initial HDO product stream. Because oxygenates are typically highly polar, the unreacted oxygenates tend to partition preferentially into the water phase of the initial HDO product. Although the hydrocarbon portion of the initial HDO product can be subjected to additional HDO polishing or separation, the aqueous portion containing large amounts of unreacted oxygenates typically is either discarded or subjected to an energy intensive separation of water from the oxygenates.
Subjecting the aqueous HDO product to additional HDO is extremely difficult due to the elevated water content which increases the heating and cooling requirements, damages the HDO catalyst, lowers the hydrogen partial pressure during subsequent HDO, and increases the required size and cost of the second HDO reactor.
Accordingly, a need exists for systems that allow more efficient HDO of oxygenates without increasing the severity of the process.